Local oscillator circuit tuning device



March 22, 1938. w. VAN B. ROBERTS 219737 LOCAL AOSCN rLnAQR CIRCUITTUNING DEVICE A i Original Filed Oct. 17, 1934 2 Sheets-Sheet 1ATATORNEY March 22, 1938.,

W. VAN B. ROBERTS 2,111,731?

LOCAL OSCILLATOR CIRCUIT TUNING DEVICE I Original Filed Oct. 17, 1934 2Sheets-Sheei 2 ATTO R N EY Patented Mar. 22, 193s UNITED STATES LOCALosCILLA'roR CIRCUIT TUNING DEVICE Walter van B. Roberts, Princeton, j.,as-

signor to Radio Corporation of America, a

corporation of Delaware Application October 17, 1934, Serial No.'748,609A

`Renewed. March 24, 1936` 2 Claims. (C1. 25o-20) My present inventionrelates to tuning devices for superheterodyne receivers, and moreparticularly to a method of, and means for, automatically insuringand/or indicating exact tuning of that type of receiver.

The superheterodyne type of radio receiver is a highly selectivereceiver of broadcast signals. Tuning of such a receiver has beenaccomplished by the set operator by utilizing the ear to determinewhether the set is sharply tuned to the desired carrier frequencyl vTuning meters have been proposed as a substitute for the operators ear.In the case of sets equipped ywith automatic volume control a mechanicalsubstitute for the ear is essential. However, such tuning metersemployed in the past have generally been unsatisfactory becausethey havenot indicated the direction and magnitude of detuning.

It is one `o1. the primaryobjects of this invention to provide a tuningindicator for a radio receiver of the superheterodyne type, theindicator being constructed to exactly register-a resonant condition ofthe receiver, and, additionally, to indicatethe direction and frequencymagnitude off resonance when the receiver is detuned.

Another important object of the invention may be said to reside in atuning meter construction for a superheterodyne receiver which comprisesa pairY of rectifier circuits, tuned equal frequency amounts to eitherside of the operating intermediate frequency, whose outputs are balancedat the intermediate frequency, an indicator responsive tothe Vdirectcurrent output of the rectier circuits being utilized to indicate theresonance condition of the receiver.

Another problem is encountered in superheterodyne receiver operation.vThe local oscillator circuit of the receiver is tuned through afrequency range differing at all times from the signal circuit frequencyrange by the operating intermediate frequency. For one reason or anotherthe predetermined relation between oscillator frequency and tuning dialsettings may vary. In general, let it be assumed that it is difficult tosecure exact tuning by mere adjustment of the manually adjustable tuningdevice.

Accordingly, it is pointed out that another important object of myinvention is to provide a novel method of, and means for, automaticallyinsuring exact tuning of a superheterodyne receiver wherein variationsin frequency of the converter output of the receiver result inreactanceadjustments in the local oscillator network.

Another object of the invention is to provide in combination with thelocal oscillator network `of a superheterodyne receiver, a frequencyconto .provide tuning indicators and/or automatic local oscillatortuning-means for such receivers Whichlare not'only dependable inoperation, but economically assembled in the receivers.

yThe novel fatureswhich IV believe to be charactertistic. of myinvention are set forth in particularity in *the appended claims, theinvention itself, however, "as to -both its organization and method ofoperation will best be understood by reference tothe followingdescription taken in connection withthe` drawings in which I haveindicated diagrammatically one circuit organization whereby my inventionmay be carried into i effect.

In the drawings:-

Fig. 1 illustrates a circuit diagram of a superheterodyne receiverembodying the invention,

Fig. 2 graphically illustrates the operation of the indicator andfrequency control mechanism of Fig. 1,l

Fig. 3 is a schematic representation of a mechanical embodiment of anindicator and frequency changer mechanism embodying the presentinvention. 1- 1 ,Y

Referring now to the accompanying drawings, wherein like referencecharacters designate similar circuit elements in the different figures,there is shown in Fig. 1 inpurely schematic manner a superheterodynereceiver of the type including a signal collector, such as a groundedantenna circuit A, a radio frequency amplier l, rst detector, anintermediate frequency amplier and a second detector. The radiofrequency amplier I may comprise one, or more, stages of tuned radiofrequency amplification. Each amplifier, as is well known to thoseskilled in the art,.may include a tube of the screen grid or pentodetype, or even of the variable mu type. The numeral 2 is to be understoodas symbolizing the variable tuning condensers employed for tuning anamplifier of the tuned radio frequency type. Thev signal collector A iscoupled, as at Ma, to the tuned inputcircuit of the amplier I.

The first detector is shown including a screen grid tube 3 whichincludes a tunable signal input circuit, and in the input circuit isprovided a variable tuning condenser 5 of the same type as the condenser2. The plate circuit of tube 3 includes a resonant circuit 6, tuned tothe operating intermediate frequency, the circuit being magneticallycoupled, as at Ms, to the resonant input circuit of the intermediatefrequency amplifier il. The amplifier 8 may include one, or more,`

stages of amplification, and it will be understood that the input andoutput circuit of each of these amplifiers is maintained xedly tuned tothe operating intermediate frequency.

The second detector 9 has its resonant input circuit Ill coupled to thetuned output circuit II of amplifier 8 through the couplingl Ms. Thedetected output of the second detector is to be utilized in any desiredmanner, and this may be accomplished by impressing the detected outputupon an audio frequency amplifier, of one or more stages, which isfollowed by a reproducer; Locally produced oscillations, of a frequencyintended to differ at all timesA from the frequency to which the signalcircuits are tuned bythe operating intermediate frequency, are producedby the tube I2 and its associated circuits. The local oscillator tube I2has the resonant circuit I 3 connected between its grid electrode andcathode. The circuit I3 includes the variable tuning condenser I4. Theplate circuit of tube I2 is magnetically coupled to the input circuitI3, as by coupling M2, and the rotors of condensers 2, 5 and I4 arearranged for mechanical uni-control, shown by the dotted lines in Fig.l.

The uni-control device is designated as a manual tuner, and it will beunderstood that the condensers I4, 5 and 2 are mechanically arranged ina gang condenser; these variable units are preferably similar. Thefrequency of thelocal oscillator is maintained substantially constantlydifferent from the frequency of the signal circuits by means of thepadding condensersV I5 and I6. The condenser I5 is arranged in serieswith the tuning condenser I4, and the condenser I6 is arranged in shuntwith the series condensers lI5 and I4. The magnitudes of-condensers I5and E6 are so chosen with respect to the magnitude of condenser I4 thatthe frequency of the local oscillator will differ from the frequency ofthe signal circuits by the operating intermediate frequency at anysetting of the manual tuner. It is not believed necessary to explaininany further detail the relations between condensers I4, I5 and I5since this is well known to those skilled in the art, and has moreoverbeen fully described by W. L. Carlson in U. S. Patent 1,740,331 ofDecember 17, 1929.

The oscillations produced by the local oscillator are impressed upon therst detector in any well known manner, as for example through thecoupling M1. The various circuit elements described to this point areall conventional; those skilled in the art are fully aware of thedetails of construction of such a superheterodyne receiver, andsufficient has been stated in connection with such a receiver to showfully the advantages of the present invention, Which will now bedescribed in detail. A pilot, or'control, network is provided, and thiscontrol network includes a pair of resonant circuits 20 and 2 I. Circuit25 comprises the coil 22 shunted by condenser 23, and circuit 2Iincludes the coil 24 sli-unted by condenser 25. The low alternatingpotential sides of circuits 20 and 2I are connected together andgrounded. The high alternating potential sides of circuits 20 and 2I areconnected together through a pair of alternating current rectiers 25 and21, the rectiers being arranged in series relation. These rectiers maybe, for example, of the copper oxide type, and are devices well known tothose skilled in the art; they are elements possessing asymmetricalconductivity. The junction of the rectifiers 26 and 2l' is connected tothe grounded side of circuits 20 and 2I through meter G having a scale28 and a movable indicator needle 29. 'I'he meter G is shunted by aradio frequency by-pass condenser The circuits 25 and 2l are tuned tofrequencies on either side of the operating intermediate frequency by anequal amount. Thus, if the operating intermediate frequency is 175kilocycles, then circuit 20 is tuned, for example, to a frequency of 173kilocycles, and circuit 2| is tuned to a frequency of 177 kilocycles.The circuits 20 and 2I are, moreover, magnetically coupled to one of theintermediate frequency circuits, such as circuit II, by a couplingdevice desired to be employed, and conventionally represented by thesymbols M3 and M4. In other words, energy having a nominal frequency of175 kilocycles is impressed upon the circuit 20 which is tunedsubstantially two kilocycles to one side of the desired 175 kilocycleintermediate frequency, and intermediate frequency energy is alsoimpressed upon circuit 2I which is tuned substantially 2 kilocycles offresonance to the other side of the desired intermediate frequency.

The couplings M3 and M4 are equal, and the circuits 20 and 2| are assharply tuned to their respective frequencies ask possible. Let it beassumed, for the moment, that the dotted line coupling 45 is absent fromthe receiver system; the function of this dotted line coupling will beexplained at a later point. From Fig. 2, which is a graphicrepresentation of the current relations in the control network, it willbe seen that if the manual tuner is adjusted so as to produce a beatfrequency lower than the operating intermediate frequency of 175kilocycles, then the tmeter G will deflect in one direction.' On theother hand, if the manual tuner is adjusted to produce beats higherthan-175 kilocycles the meter G will deflect in the other direction.When the tuning is correct, the meter G does not deflect at all. Also,if the tuning is so far off that only small amounts of current pass theintermediate frequency circuits, the meter G will not be appreciablydeflected. Thus, the meter indication will show inthe vicinity of propertuning whether the manual tuner is on one side or the other of theproper tuning adjustment. The correct adjustment causes the meter toindicate zero regardless of the signal strength. Hence, the meterconstitutes an ideal tuning indicator.

Curve X of Fig. 2 represents the current flow in the 173 kilocyclescircuit 20, frequency of beats being plotted against current. The curveY denotes'the current in the 177 kilocycles circuit. Thek dotted linecurve Z represents the direct current flow through the meter G asthebeat frequency varies. The construction of the meter G is not shownin detail because those skilled in the art are fully aware of themechanical construction of such a meter. Merely by way of reference itis pointed out that the meter G can be any type of galvanometer whichincludes the usual magnetic poles.

A current equal to the 75" difference between the rectified currentoutputs of rectiers 26 and 21 flows through meter G, whose indicator, orneedle, 29 will be deflected thereby in a sense determined by thedirection and magnitude of the difference current. If, for example, themanual tuner is adjusted to a point such that the beat energy impressedon circuit il is less than 175 kilocycles, the needle 29 will bedeflected to the right, and the amount of deflection will be anindication of the kilocycles off resonance. This eifect arises becausewhen the beat frequency is below 175 kilocycles, the 173 kilocyclecircuit 20 will be more nearly initune with such beat frequency, andwill cause a flow of direct current through the galvanometer G, and thiswill deflect the needle 29 to the right. Y

When thebeat frequency rises above 175 kilocycles the circuit 2| will bemore nearly in tune with such beat frequency, and willicause the needle29 to swing to the left of the zeroV point of the meter G. In this waythe user of the set is not only able to see at a glance,` and withouthaving to listen to the loud speaker, Ywhether or not the receiver is intune to the desired station, but, if not, in which direction the tuningshould be readjusted.` Furthermore, if automatic volume control is usedso that the output of intermediate frequency amplier 8 is substantiallyconstant, thenumber of division deections of meter G will beproportional to the amount by which the I. F. departs from the desiredvalue; at least when the tuning is near enough to being correct so thatsignals are heard with normal Such a visual tuning indicator is ofparticular value in a receiver utilizing automatic volume control. Anautomatic volume control network is not shownin the circuit diagrambecause those skilled in the art are fully aware of such volume controlcircuits, and can readily find information for the construction of sucha'volume control circuit. Reference is made to my Patent No. 1,913,959of June 13, 1933, for an automatic volume control arrangement which maybe employed in a superheterodyne receiver system of the type shown inFig. 1. As is well known, the use of automatic volume control in a radioreceiver makes it diflicult to tune the receiver by ear. The utilizationof a visual tuning indicator of the type disclosed in this applicationwill facilitate the tuning of a receiver employing automatic volumecontrol.

'I'he control network which includes circuits 20 and 2l may further beutilized for automatically tuning the receiver. This is readilyaccomplished by constructing the shunt condenser I6 so as to include astator and a rotor plate. The rotor of condenser I6 may be mechanicallycoupled, as shown by dotted line 49, to the indicator of the meter G. Itis only necessary to correlate the movement of the indicator of meter Gwith the displacement of the rotor of condenser I6 to secure tuning ofthe local oscillator network. In Fig. 3 is shown an enlarged view ofsuch a mechanical correlation of the indicator mechanism of meter G andthe condenser I6. In this gure the rotor l of the shunt condenser i6 isshown mechanically fixed to the indicator mechanism of meter G so thatwhen the indicator needle 29 moves to one or the other side of the zeroindication of scale 2B, a predetermined displacement will be securedbetween the stato-r I6 and the rotor I6' of the local oscillator shuntpadding condenser.

Itwill be observed that the scale of the meter in Fig. 3 is calibratedin kilocycles off resonance below and above the operating intermediatefrequency of 175 kilocycles. The zero mark indicates that the manualtuner has ben adjusted to tune in the desired station, and that the beatfrequenoy is equal to the intermediate frequency of 175 kilocycles. Whenthe needle 29 indicates zero on the scale, the relation between thestator and rotor of condenser I6 is substantially as shown in Fig. 3.Let it be assumed that the manual tuner is adjusted to a stationsetting. If the tuning adjustment is incorrect, and such that the beatfrequency is less than 175 kilocycles, the needle 29 will swing to theright along the scale and to such a point as to indicate the number ofkilocycle that the receiver is below resonance. Y Simultaneously, andfrom Fig. 3 this will be clear, it will be seen that the rotor I6' isdisplaced to the left, and the capacity value of condenser |5 isreduced. This reduction in value of condenser I6 results in a rise inthe frequency of the local oscillator. Of course the relation betweencapacity changes of condenser I6 and the movement of needle 29 is apredetermined one. For any given accurate setting of the manual tunerthe zero setting of the condenser I9 is that setting at which the localoscillator network is correctly tuned. When the needle 29 swings to theleft on the scale, the rotor I6' will swing to the right and increasethe capacity value of condenser IB, and thus decrease the localoscillator frequency at the particular setting of the manual tuner.

The mechanical linkage 40,'generally shown in Fig. 1, is adjusted sothat at zero meter deection the small shunt capacity l5 is of the propervalue to `make the local oscillator track with the radio frequencysignal tuning. If the manual tuner is not set correctly the meter Gdeects, and changes the value of the condenser I6 in such a direction asto change the local oscillator frequency in that sense which makes thebeat frequency approach more nearly to the desired value of 175kilocycles. As the beat frequency approaches 175 kilocycles the currentthrough the galvanometer meter G decreases, so that at a certain amountoff tune the galvanometer current just balances the restoring force(which may be the well known restoring spring of the galvanometer of themeter). By making this restoring force small, and the change of capacityof condenser I6 with respect to movement of the mechanical linkagelarge, this equilibrium position can be made to occur at a beatfrequency differing very little from the desired intermediate frequencyof 175 kilocycles.

Thus, it will be seen that if the manual tuner will be set to anyposition close enough to the correct position to bring in the signal atall, the automatic tuning action will function to bring thetuning to asufficiently close value to avoid detuning distortion. This assumes, ofcourse, that the radio frequency selectivity is not suiciently sharp tocause detuning distortion when the manual tuner is set closely enough tobring in the signals through the highly selective intermediate frequencysystem at all.

It will be obvious to one skilled in the art that damping means may beincluded in the meter G to prevent swinging hunting if such occurs. Suchdamping is often included in galvanometers, and is usually electrical,although it is also possible to arrange the moving part of G to move ina viscous medium, or air, in such a way as to introduce into the systema frictional resistive force proportional to velocity.

While I have indicated and described several systems for carrying myinvention into effect, it Will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In combination in a superheterodyne receiver of the type including atunable signal circuit and a single tunable local oscillator circuit,means for simultaneously tuning both signal and oscillator circuitsthrough frequency ranges which constantly differ by a predetermined beatfrequency, an auxiliary network comprising a pair of resonant circuits,each resonant circuit including a rectifier, means for impressing thebeat frequency energy upon said resonant circuits, one of said resonantcircuits being tuned to a frequency differing from the beat frequency bya predetermined frequency value, the other resonant circuit being tunedto a frequency differing from the beat frequency by the same frequencyvalue but in an opposite direction, mean's for utilizing thedifferential direct current output of said rectifiers for indicating themagnitude and direction of flow of the rectified currents, an auxiliarytuner device electrically connected to the oscillator circuit, and meansresponsive to operation of the indicating means for actuating saidauxiliary device.

f 2. A radio receiving system comprising means for collecting amodulated carrier Wave, a first detector having an output circuit, alocal oscillator coupled to said first detector for heterodyning saidcarrier Wave to an intermediate frequency signal, said oscillatorincluding a variable reactance device for varying its frequency, atuning device mechanically connected to said variable reactance wherebythe frequency of said oscillator may be varied by moving said tuningdevice to adjust the magnitude of the reactance, an auxiliary adjustablereactance electrically connected with said first reactance device, saidauxiliary reactance comprising a variable condenser having a stator androtor, an intermediate frequency utilization network coupled to theoutput circuit of said first detector `and tuned to a predeterminedintermediate frequency, and means for holding the carrier frequency of'said intermediate frequency signal substantially constant in the regionof said predetermined frequency While moving said tuning device througha predetermined distance, said frequency holding means comprising anelectro-mechanical device, said last device including an adjustableelement adapted to be displaced, a visual resonance indicator includingsaid adjustable element as an indicator, the rotor of said auxiliaryreactance 'being aflixed to said adjustable element for displacementtherewith as a unit, means for deriving a uni-directional current fromsaid intermediate signal When the latter shifts in frequency from saidpredetermined frequency, and said electro-mechanical device beingconnected to said deriving means so as to be responsive to saiduni-directional current.

WALTER VAN B. ROBERTS.

